1. Field of the Invention
The present invention relates to an electrophotographic imaging apparatus, and more particularly, to a compact collimation assembly providing for alignment of adjacent laser light sources relative to collimation lenses in an electrophotographic imaging apparatus.
2. Description of Related Prior Art
In electrophotography, a latent image is created on the surface of an electrostatically charged photoconductive drum by exposing select portions of the drum surface to laser light. Essentially, the density of the electrostatic charge on the surface of the drum is altered in areas exposed to a laser beam relative to those areas unexposed to the laser beam. The latent electrostatic image thus created is developed into a visible image by exposing the surface of the drum to toner, which contains pigment components and thermoplastic components. When so exposed, the toner is attracted to the drum surface in a manner that corresponds to the electrostatic density altered by the laser beam. Subsequently, a print medium such as paper is given an electrostatic charge opposite that of the toner and is passed close to the drum surface. As the medium passes the drum, the toner is pulled onto the surface of the medium in a pattern corresponding to the latent image written to the drum surface. The medium then passes through a fuser that applies heat and pressure thereto. The heat causes constituents including the thermoplastic components of the toner to melt and flow into the interstices between the fibers of the medium and the fuser pressure promotes settling of the toner constituents in these voids. As the toner is cooled, it solidifies and adheres the image to the medium.
Further, color laser printers typically employ one light source and optical path for each of a plurality of latent images to be simultaneously formed on the drum. For a color tandem printer, four distinct laser scanning units are typically required, each with its own laser diode light source, polygonal scanning mirror and associated motor, and optical system. Generally, the largest and most costly components of laser scanner units are the motors for driving the polygonal mirrors and the lens sets. Accordingly, in order to reduce costs and reduce the size of the printer and increase the reliability of the printer, the concept of scanning multiple laser beams with a single scanning mirror has been implemented.
A typical polygonal mirror for use in a multi-beam scanning unit typically has a height dimension of no more than about 2 mm at the reflective facets of the mirror, and laser diodes for such applications are typically mounted in a cylindrical housing having an outer diameter dimension greater than 5 mm. In order to image multiple imaging beams onto a single polygonal mirror simultaneously, for example, by positioning light sources adjacent to each other in a cross-scan direction, it is necessary to direct the beams onto the mirror facets at some non-parallel angle relative to the axis of rotation of the polygonal mirror. However, as this angle becomes larger, the error caused by facet to facet manufacturing tolerances of the mirror creates a shift in the focal location of the image formed at the photoconductive drum, resulting in a print quality defect. Accordingly, it is desirable to position the adjacent light sources and corresponding collimation lenses with a spacing in the cross-scan direction that is as close as possible, while maintaining a capability to adjust the axes of the light beams to direct the light beams to predetermined locations relative to the polygonal mirror.